1. Field of the Invention
The invention relates to an annular workpiece quenching method and a quenching apparatus used in the method.
2. Description of Related Art
High-carbon steel (hereinafter, referred to as “bearing steel”) that exhibits temperature-dimension change characteristics as shown in FIG. 4 during heat treatment, is used for an annular member such as a bearing ring of a rolling bearing. The rolling bearing is manufactured by subjecting an annular workpiece made of the bearing steel to heat treatment such as quenching in order to obtain a desired mechanical strength. However, when the workpiece is subjected to quenching, the workpiece undergoes dimension change during the quenching, due to stress release and thermal expansion during heating and due to thermal contraction and transformation expansion caused by martensitic transformation during cooling. If the timing of the dimension change varies in the workpiece, distortion occurs, which may cause reduction in a roundness of the workpiece and dimensional variations.
In order to improve dimensional accuracy of inner and outer diameters of the heat-treated workpiece and reduce thermal distortion of the heat-treated workpiece, a heat treatment method described in Japanese Patent Application Publication No. 2010-248556 is proposed. In this heat treatment method, a workpiece is cooled in a state where an inner die is arranged radially inward of the workpiece that has been heated, and an inner peripheral surface of the workpiece undergoes thermal contraction to make contact with an outer peripheral surface of the inner die. In this way, the inner peripheral surface of the workpiece is restrained. Then, when a temperature of the workpiece becomes equal to or lower than a martensitic transformation start temperature in a cooling step, the workpiece is removed from the inner die, and then, the workpiece is inserted in an outer die such that an outer peripheral surface of the workpiece faces an inner peripheral surface of the outer die. In this state, the workpiece undergoes volume expansion due to the martensitic transformation, and the outer peripheral surface of the workpiece makes contact with the inner peripheral surface of the outer die. As a result, the outer peripheral surface of the workpiece is restrained.
In the above-described heat treatment method, when the temperature of the workpiece becomes equal to or lower than the martensitic transformation start temperature, the workpiece is removed from the inner die and is then inserted into the outer die to change a workpiece restraint state from a state where the inner peripheral surface of the workpiece is restrained to a state where the outer peripheral surface of the workpiece is restrained. However, if the timing of this change delays, in some cases, an outer diameter of the outer peripheral surface of the workpiece exceeds a predetermined outer diameter due to the volume expansion caused by the martensitic transformation, and, as a result, the workpiece is inserted into the outer die while the outer peripheral surface of the workpiece makes contact with the inner peripheral surface of the outer die. In this case, the outer peripheral surface of the workpiece is scratched due to contact with the outer die. As a result, an amount by which the workpiece is ground increases, which may prolong a cycle time in a grinding step.